1. Field of the Invention
This invention relates to a digital signal measurement apparatus adapted for measuring a signal-to-noise ratio in a measured device such that a digital signal is inputted thereto and is outputted therefrom.
2. Description of the Prior Art
As a conventional configuration of a digital signal measurement apparatus adapted for measuring a signal-to-noise ratio (S/N) in a measured device adapted so that a digital signal is inputted thereto and is outputted therefrom, there is a measurement apparatus to which, e.g., a method of measuring a signal-to-noise ratio described in the Japanese Laid Open Patent Application No. 112313/90 is applied.
Namely, the measurement method described in the above publication comprises the steps of applying a sine wave signal to a measured circuit, sampling outputs from the circuit to store them into a memory, eliminating an offset component included in the content of the memory, carry out phase adjustment with the sine wave signal with a zero crossing point of the content being an initial point by using an interpolating function based on the sampling theorem, sampling phase-adjusted signal data, determining a sum of squares deviation between the sampled signal data and the sine wave signal subjected to gain corrective operation, and determining a signal-to-noise ratio by the result of the sum of squares deviation.
Accordingly, the configuration to which the measurement method described in this publication is applied is as shown in FIG. 1. The configuration of FIG. 1 will be briefly described below. In FIG. 1, a sine wave signal outputted from a sine wave generator 11 of a measurement instrument 3 is sent to a filter 2 of a measured circuit through terminals 12 and 21. An output of the filter 2 is sent to a memory 14 through terminals 22 and 13, and is stored thereinto. An output of the memory 14 is sent to an offset canceler 31, at which an offset component included in the output of the memory 14 is eliminated. An output of the offset canceler 31 is sent to an operational element 32, at which a zero crossing point is calculated. Further, at the operational element 33, a phase adjustment with the sine wave signal is carried out by using the zero crossing point as an initial point. Thus, phase-adjusted data is sent to an adder 36.
On the other hand, the sine wave signal from the sine wave generator 11 is also sent to a variable gain amplifier 34. At the variable gain amplifier 34, a correction corresponding to the gain of the filter 2 is applied to the sine wave signal. The gain-corrected signal thus obtained is sent to the adder 36 through the operational element 35. At the adder 36, a sum of squares deviation between the gain-corrected sine wave signal and the phase-adjusted data is determined. On the basis of an output of the adder 36, a S/N is determined at an operational element 37.
Here, the measurement method described in the above publication has problems as mentioned below.
Namely, first, since calculation of noise is made merely by comparison between digital data, e.g., in the case where a measured circuit (measured system) is a linear system, S/N does not correspond with that in the case of an analog measurement. In other words, in the case where a measured circuit (measured system) is a linear system, a measured result in the digital region and a measured result of the analog measurement corresponding to a bit length in the measured circuit do not correspond with each other and compatibility is lost.
In addition, in the configuration to which the method described in the above publication is applied, gain correction (e.g., gain correction at the variable gain element 34) is applied to a sine wave signal of a measurement signal by taking a gain of the measured circuit into consideration. For this reason, there is the possibility that an error of this gain correction may affect the measurement of the S/N.